1. Field of the Invention
The invention relates to a method of measuring ionic mobility, wherein the ions in a medium are carried by means of an electric field and their mobility is measured. The invention also relates to a device for measuring the ionic mobility, comprising an electric field for carrying ions in the medium and an ion meter for measuring the ionic mobility.
The measuring device of ionic mobility herein refers to a device that is used for measuring the ionic mobility in the medium and the mobility distribution in the electric field. Generally, the medium that is measured by these devices, in practice, a gas, is ionized before the actual measurement, for example, by using a radioactive source, a corona discharge or ultraviolet light.
2. Description of the Related Art
One typical method of implementation of the device comprises a drift tube, which examines the time taken by the ions in the gas when to move a given distance under the effect of a given electric field. As ions that have different mobilities take different times travelling the distance, a mobility spectrum is obtained by means of this method. U.S. Pat. No. 4,777,363 and WO 89 09934, among others, describe the basic technique of the drift tubes. In addition, there are numerous patents that present various improvements to this technique. Typically, the technique uses a closed cycle, wherein only part of the gas that is examined is allowed to influence the actual measurement.
Another way to examine the ion mobility in gas is to use a solution of the aspiration condenser type, wherein the ions are deflected by an electric field that is perpendicular to the flow direction of the gas. Their mobility in the electric field determines, how far the ions fly before colliding either with walls or collector electrodes placed on the walls. The ions that collide with the collector electrodes generate a measurable flow, from which the ion mobility distribution of the measured gas can be deduced at a given accuracy. Typically, the ionized gas that is examined by this technique is allowed to travel as such through the measuring device. Patent FI75055, among others, describes this technique. Patent WO 03 081224 describes slightly different ways of implementing this type of cell structure; among others, a more precise way of measuring the mobility distribution by means of a reference electrode and an alternating field voltage. Applications US 2003/0047681 A1 and US 2005/0178962 A1 disclose the axial flow of a sample medium between two parallel cylindrical surfaces of an FAIMS analyzer. A challenge posed by these techniques is to equalize the gas flow travelling through the cell, among others. Furthermore, because of the open structure of the cell, any variations in the ambient temperature and, thus, also the temperature of the gas that is measured have a direct effect on the measured signal. In some cases, the laminarity of the flow and the service life of the pumps that are used for pumping the gas also pose a problem. The typical noise level of the pumps is also quite high.